Call forwarding is one of the most commonly used services in modern telecommunication networks. A subscriber can specify that calls destined to him are to be forwarded to another subscriber's number. For example, Beth can transfer her calls to Cindy when leaving her home to visit Cindy. A subscriber can specify that his calls be forwarded in various situations. For example, when the CFU (Call Forwarding Unconditional) service is activated, all the calls destined to the subscriber are forwarded to a given number. In the CFB (Call Forwarding on Busy) service, calls are forwarded if the called subscriber is busy. By activating the CFNRy (Call Forwarding on No Reply) service, the subscriber instructs that his calls are to be forwarded after 4 alert tones, for example, if the call is not answered before that. For mobile subscribers the CFNRc (Call Forwarding on Not Reachable) service can be defined. In CFNRc the calls are forwarded if the subscriber is not in the coverage area of the mobile telecommunication network, or if he has turned off his mobile station.
The routing of forwarded calls, as well as the charging principle for forwarded calls, are shown in FIG. 1. In the figure, the calling party A makes a call to B. B has forwarded his calls to C. This is done by giving the exchange of B instructions to forward the calls addressed to B to a third party C. Both B and C are fixed network subscribers. A may be a fixed network subscriber or a mobile network subscriber. The call is first routed from A via his Local Exchange LE(A) to the local exchange LE(B) of B. Information concerning the specified call forwarding is stored in LE(B). In response to having received the request for call establishment from LE(A), LE(B) notices that the call is to be forwarded to C. In response to that, LE(B) routes the call to the forwarded to party C via the local exchange LE(C) of C. LE(B) becomes the transit point of the call. The first call leg from A to LE(B) is paid by subscriber A. The forwarded leg from LE(B) to C is paid by subscriber B.
In FIG. 2 is shown a situation where the call is forwarded to mobile subscriber. In this example, both A and B subscribers are fixed network subscribers. However, they can also be mobile subscribers without causing any remarkable changes in the process. When compared to the case shown in FIG. 1, the only difference is the leg between LE(B) and subscriber C. This is due to the basic feature of mobile telecommunication systems: the location of the subscriber is not fixed. Therefore, the location of the subscriber has to be ascertained before establishing the connection.
In modern mobile telecommunications networks, the network maintains information about the location of the mobile subscribers. In the GSM network used as an example in this application, this information is distributed between the home location register HLR and visitor location registers VLR directly connected to mobile services switching centers MSC. The HLR is in the home network of the mobile subscriber and contains the permanent subscriber information and the location information of the mobile subscriber with an accuracy of one VLR area. The VLR area typically equals the area served by one mobile services switching center. The VLR of the visited mobile services switching center VMSC responsible for the area the subscriber is currently visiting contains more exact information about the subscriber's location.
On the leg from LE(B) to subscriber C, the call is first routed to the gateway mobile services switching center GMSC(C) of subscriber C. GMSC(C) is basically any MSC capable of making interrogations to the home location register HLR(C) of subscriber C. In response to having received the HLR inquiry, i.e. the query for routing information, the HLR ascertains the VLR the subscriber is currently visiting, and sends a request PRN (Provide Roaming Number) for a roaming number to that VLR, i.e. VLR(C).
In response VLR(C) returns HLR(C) a roaming number MSRN (Mobile Subscriber Roaming Number) identifying the called subscriber in this VLR. HLR(C) forwards the roaming number to GMSC(C). GMSC(C) adds to the call record comprising information of the call information that subscriber C is receiving the call in the area of VLR(C). Using the received identification MSRN and the message IAM(MSRN) (IAM=Initial Address Message), GMSC(C) establishes a connection to the MSC VMSC(C) currently visited by subscriber C. To identify the called subscriber corresponding to the received roaming number with his identity number IMSI or TMSI (Temporary Mobile Subscriber Identity) VMSC(C) makes a query to VLR(C), which then returns this information. According to the specifications, this is done using messages SIFIC (Send Info For Incoming Call) and CC (Complete Call). Having ascertained the identity of the called subscriber, VMSC(C) now establishes the final leg of the call to the called subscriber.
In GSM, the basic principle in charging is that the calling subscriber pays the leg between himself and the home network HPLMN(B) (HPLMN=Home Public Land Mobile Network) of the called subscriber. The rest of the route is paid by the called subscriber. The idea here is that the calling subscriber should not have to pay a surprisingly high price for his call, for example, when the called subscriber is roaming in a distant country. According to this principle, when subscriber A in Finland calls a mobile subscriber B whose home network is in New Zealand but who is currently roaming in Finland, the call is routed via New Zealand. Subscriber A pays the leg from Finland to New Zealand and subscriber B the leg from New Zealand back to Finland.
Due to this charging principle, the forwarding subscriber B pays for the leg between LE(B) and the home network of C and subscriber C from his home network onwards. It should be noted that for most operators, reception of calls is free in the home network HPLMN (Home Public Land Mobile Network) of the subscriber. However, when the subscriber is roaming outside his home network, he is charged for the roaming leg from his own home network to the VPLMN (Visited Public Land Mobile Network) he is currently visiting.
The call transfers may be annoying to the subscriber to whom the calls have been transferred especially if he is roaming abroad. An example of such an annoying situation is a malicious call forwarding. In addition to malicious call forwarding, calls may be transferred to a wrong number because of a type mismatch when entering the call transfer command to transfer all the incoming calls of the exchange of a company, for example. The subscriber to whom the calls are forwarded to may, of course, define all his incoming calls to be barred. However, the use of barring of incoming calls prevents him from receiving the calls directed to him directly, i.e. without the malicious or accidental forwarding defined by the forwarding party.
WO98/05153 teaches a method for barring calls forwarded from a certain number. The barring allows a user to selectively to prevent calls from incoming, if they are forwarded from a certain pre-defined number. This provides a solution to a problem of selectively barring calls forwarded by a certain forwarding number.
The objective of this invention is to solve the above-mentioned problem of the prior art. This objective is achieved using the method and apparatus specified in the independent claims.